The invention relates to a device intended for the analysis and for the reading of an optical signal of the type which can be analysed as at least one fundamental wave train associated with a set of secondary wave trains, which emanate from said fundamental train and which exhibit a set of corresponding delays in relation to said fundamental wave train.
The general principle of reading information comprises delaying a reference signal, namely, for example, the fundamental wave train, in order to cause an interference of said reference signal with the signals containing the information to be analysed, namely, for example, each one of said secondary wave trains.
Said information to be analysed may be, for example, the distance between the wave trains or alternatively the amplitude of the secondary wave trains.
This type of signal is, for example, obtained at the exit of a monomode fiber with maintenance of polarization, which is associated with a linearly polarized source 11 of broad spectrum (or a source emitting a beam which is linearly polarized outside the source), the fiber comprising points of coupling 12 between the two birefringent axes (FIG. 1). The optical sensor per se is constituted by the fiber section comprising the points of coupling 12, and may be either intrinsic (that is to say the fiber itself) or extrinsic (in the form of an external element).
The points of coupling 12 are, for example, constituted by the introduction of a slight shift in rotation of the axes of birefringence within the fibre, at each point c.sub.1, c.sub.2, . . . c.sub.n. Each one of these shifts, which are, in general, small in order to avoid multiple couplings, introduces a perturbation of the polarized fundamental wave train 13 emitted by the source 11. Each perturbation thus generates a secondary wave train 15 in polarization orthogonal in relation to the fundamental wave train 13-14. In other words, assuming the entrance wave train 13 coupled in the fast mode, there is recovered at the exit:
a fundamental wave train TO.sub.f 13, which has remained in the fast mode, and of amplitude e.sub.f -; and
a series of secondary wave trains TO.sub.i 15, which are coupled in the slow mode, and of amplitude e.sub.i.
It is then possible to project all the wave trains 14-15 in the same state of polarization, by means, for example, of a polarizer at 45.degree. on the axes of birefringence.
Such a sensor with multiple points of coupling 12 may, for example, be used as continuous temperature probe, by utilizing the heat-sensitive properties of the propagation velocity differential between the two axes of birefringence of the optical fiber. In this case, the analysis and the reading of the signal received from the sensor consists in measuring the effective delay of each secondary wave train 15 in relation to the exit fundamental wave train 14. In a known manner, such a reading is undertaken by retarding the fundamental signal in the reading device, until detection of its interference with each one of the secondary wave trains. The value of the delay which is read permits the calculation of the temperature, after calibration of the device.
In a known manner, the means which is simplest (at least conceptually) for performing such a reading consists in using a Michelson interferometer which can be scanned, in order to induce the delay which is necessary in order that the fundamental wave train should be able to interfere with the wave trains TO.sub.i.
However, if consideration is given to a sensor comprising 100 points of coupling which are distributed with an interval of 10 m between each coupling and a fiber the birefringence of which is 5.multidot.10.sup.-4, the scanning of the interferometer must be 50 cm. Having regard to the necessary mechanical precision, such a scanning range involves a relatively low reading frequency and a limited flexibility of use: the reading can be only with sequential access (wave train N1, then N2, then N3 etc . . . ).
A second method which is suitable for this type of device would consist in processing the various wave trains in parallel (FIG. 2a). However, in this case, the number of detectors must be of the order of the number of couplings. In order to alleviate this disadvantage, it is possible to use active couplers. In this case again, the number of couplers must be of the order of the number of points of couplings (typically approximately 100), irrespective of the parallel or series structure adopted.
Furthermore, the series structure (FIG. 2b) exhibits the disadvantage of attenuating (on account of the repeated passages within the couplers which are switchable if, for example, they are of integrated-optics type) the secondary wave trains TO.sub.i ; this accordingly degrades the signal-to-noise ratio.